1. Technical Field
The present invention relates to selective receiver circuits and, more particularly, to a method and apparatus for selectively receiving telephone signalling tones which are present in a wide band audio signal.
2. The Prior Art
Signalling between telephone systems central offices is usually effected by multi-frequency (MF) coding. Different signalling protocols are employed in different parts of the world. For example, the inter-office MF signalling protocol used by American Telephone and Telegraph Company in the United States is referred to as R1. The R1 signalling protocol is considered to be an open loop signalling system because there is no acknowledgment of signal reception returned to the sending office. Tone pairs are transmitted at the rate of approximately seven pairs per second and reception and processing at the receiving office is assumed. There are six different tones employed in the R1 protocol, beginning at 700 Hz and ending at 1700 Hz, the tones being successively spaced by 200 Hz.
The signalling protocol employed most frequently outside the United States and Canada is the R2 protocol established by CCITT. This protocol is considered to be closed loop since reception acknowledgment codes must be received before the transmitting office sends further information. Tone pairs are sent continuously until an acknowledgment, in the form of another tone pair, is received at the transmitting office. Transmission in one direction includes pairs from a first group of six tones, successively spaced by 120 Hz, beginning at 540 Hz and ending at 1140 Hz. Transmission in the opposite direction includes pairs from a second group of six tones, also successively spaced by 120 Hz, beginning at 1380 Hz and ending at 1980 Hz.
In addition to the R1 and R2 signalling protocols, which are used to set up a call between central offices, there are other signalling arrangements, such as SF (single frequency). For example, a 2600 Hz tone is commonly employed for various supervisory signalling functions in telephony.
Multiple signalling receivers are required at each central office to separate the signalling tone from the wide band audio signals on each trunk. The R2 signalling protocol has presented particularly difficult problems for hardware designers. Since there are two groups of signalling frequencies, receivers used for receiving forwardly-transmitted signalling tones cannot be used for receiving backwardly-transmitted signalling tones. Moreover, since the individual tones are separated by only 120 Hz, discrimination is difficult, particularly among adjacent tones in the higher frequency group. When high-Q filters are employed to enhance discrimination, the response time of the filter is increased so that a tradeoff has to be made and performance is compromised.
The most advanced prior art receivers employed for the R2 signalling protocol employ analog active filters to separate tones. A threshold detector is employed at the output of each filter to determine if a signalling tone has been received. The threshold detector must have an adjustable threshold because the amplitude of the wide band audio signal in which the tones reside varies in accordance with transmission distances between central offices. In order to avoid having to adjust the threshold level, some receivers include many cascaded stages of filtering to provide sharp skirts on the filter frequency response, thereby avoiding cross-talk in the passband overlap regions. However, as noted above, this increases the filter response time, in some instances, to an intolerable degree.
In any case, the prior art filters employed for the R2 signalling protocol are extensive, require certain compromises in performance, and are useful for transmission in only one direction. Moreover, such filters are not universal since they cannot be used for other signalling protocols, such as the R1 signalling protocol, because they are permanently tuned to receive one or the other of the R2 frequency groups.